1. Field of the Invention
This invention relates to a soldering method and apparatus utilizing dual solder waves of different variable velocities, and more specifically to a method and apparatus in which solder is fed to dual solder waves at different variable velocities to achieve proper soldering of terminals and associated conductor paths while reducing crossovers or bridges (i.e., shorts) between the terminals and the conductor paths.
2. Description of the Prior Art
The advent of miniaturized electrical devices, such as those employing integrated circuits, requires that terminals and conductor paths of printed circuit boards on which the devices are to be mounted be in extremely close relationship in order to accommodate the closely spaced leads of the electrical devices. As a result, the number of solder defects in the form of solder crossovers or bridges (i.e., shorts) between adjacent printed circuit board terminals and conductor paths when the leads of the electrical devices are soldered to the terminals of the printed circuit board in a wave soldering operation, has significantly increased.
Unless these solder crossovers are detected and removed prior to testing of a printed circuit board assembly, the assembly will test as being unsatisfactory. Further, in certain instances the solder crossovers may cause overloading and damage to sensitive electrical components. Accordingly, after the wave soldering operation each printed circuit board assembly must be subjected to close visual scrutiny in one or more inspection processes in order to detect any solder crossovers which have occurred, and the detected solder crossovers must be repaired manually prior to testing. In the event that a solder crossover is not detected and repaired, resulting in damage to one or more electrical components, the components must be removed from the printed circuit board and replaced with new components in a desoldering-resoldering process.
The occurrence of solder crossover defects is particularly prevalent in one-way solder waves in which the solder flows only in a direction counter to the direction of printed circuit board movement, and in this type of solder wave is attributed to a number of factors. For example, it has been found that in this type of solder wave a printed circuit board entering the solder wave restricts the counterflow of the solder surface, while allowing the solder to flow underneath the surface, thereby leaving the surface of the solder at the critical point of board exit essentially stationary or "stagnant" during the soldering operation. As a result, the inherent high surface tension of the solder inhibits the ability of the solder to separate properly as it coalesces on the printed circuit board terminals and conductor paths. At the same time, the formation of solder alloy oxides on the "stagnant" solder surface increases and the oxides inhibit the ability of the solder to wet properly to the terminals and conductor paths. The printed circuit board also exits from the solder, which is flowing counter to the direction of board movement beneath the solder surface, at a high velocity, tending to produce excessive "dragout" of solder from the solder wave. Accordingly, the net result of the high surface tension, oxide formation and high board exit velocity is a high incidence of solder crossovers.
The use of a bi-directional solder wave, in which the solder flows both counter to the direction of printed circuit board movement and in the direction of board movement, alleviates the formation of solder crossovers to a certain extent. In this connection, the development of a stationary or "stagnant" surface condition in the solder wave at the point of board exit, as in one-directional solder waves as above described, is eliminated since the solder is continuously moving in the same direction as the printed circuit board. As a result, the surface tension of the solder and its effect from the standpoint of forming solder crossovers, tend to be reduced. Further, any oxides, to the extent that they do form, are being continuously removed from contact with the printed circuit board by the flowing solder. However, a large number of solder crossovers still are formed during the wave soldering process, apparently as a result of a high degree of motion or turbulence in the solder as a printed circuit board passes through and exits from the bi-directional wave. Accordingly, the purpose of this invention is to provide a system wherein the flow of solder counter to the direction of printed circuit board movement and in the direction of printed circuit board movement can be varied independently as may be necessary to control the solder flow in order to reduce the incidence of solder crossovers in a wave soldering operation.
An example of a known bi-directional soldering apparatus is disclosed in the S. L. Rieben U.S. Pat. No. 3,119,363, in which surfaces of the apparatus which form opposite flowing parts of the bi-directional solder wave are adjustable for varying the length, angle of entry and angle of exit of the wave. Solder is fed to the bi-directional wave from a single source and the only control of the flow of the solder counter to, and in the direction of, printed circuit board movement is that produced inherently in the adjusting of the wave forming surfaces.
The J. A. Raciti U.S. Pat. No. 3,482,755 discloses a bi-directional wave soldering machine which is intended to eliminate solder bridges or crossovers and icicles by passing an article through a moving turbulent crest portion of the solder wave, and then moving the article through a calm solder pool which is contiguous to but lower than the moving wave crest. The H. P. Eschenbrucher U.S. Pat. No. 3,565,319 is directed to a bi-directional wave soldering apparatus in which solder is fed to a pair of twin solder waves without turbulence by the use of an inner pressure tank and then passed through ducts having inner perforated baffles or plates for producing solder in the two solder waves. As in the Rieben et al. patent, in each of the Raciti and Eschenbrucher patents the solder is fed to the soldering area from a single source.